Checking the mode of function of high-integrated circuits usually occurs in computer controlled test systems, in which the extant errors can be identified by analyzing the voltage level measured at the outputs of the examined circuit in dependence on the respectively fed bit pattern, however they can only be located with great difficulty. For this reason, additional measurements must be carried out inside high-integrated circuits, particularly during the development phase.
Corpuscular beam measuring processes, especially, electron beam measuring, used in all the fields of development and fabrication of micro-electronic components have proven to be particularly suited for this purpose. With the aid of these measuring techniques, the electric potential distribution in integrated circuits can, by way of illustration, be imaged (here the processes known to those skilled in the in the art under the term "voltage coding", respectively "logic state mapping" are to be mentioned) or the temporal potential course can be determined at a single point of junction ("waveform measuring). A survey of the test procedures currently usually employed is given in the publication "Electron Beam Testing" by E. Wolfgang (perdiodical "Microelectronic Engineering", issue 4, 1986, pages 77-106). One of the most important measuring processes in electron beam measuring is so-called waveform measuring, which is described in detail in the publication "Electron Beam Testing: Methods and Applications" by H.-P. Feuerbaum (periodical "Scanning", issue 5, 1983, pages 14-24), in particular on pages 12 to 14 and with the aid of which the course of the voltage can be measured at a point of measurement of a sample.
In waveform measuring a finely focussed primary electron beam is aimed at the measuring position to be examined of the integrated circuit. The primary electrons impinging there release from the surface of the sample secondary electrons, which are influenced by the electrical potentails on the surface of the sample. This influencing manifests itself in a secondary electron stream, which is dependent on the potential at the measuring position, respectively in an energy shift of the secondary electrons, which also is determined by the electric potential at the measuring position and which can be measured with the aid of an energy spectrometer. This effect is referred to as potential contrast. As the sensors required for registering the secondary electrons usually only have a relatively small bandwidth of a few MHz, a scanning process, in which the temporal course of the signal is scanned at the measuring site for a trigger signal similar to a sampling oscilloscope with short electron pulses, has to be employed in order to achieve high time resolution. As each primary electron pulse can only contain very few electrons, an average is made of the scanning values of very many measuring cycles in order to attain an adequate signal-to-noise ratio.
As each trigger pulse releases a primary electron pulse, yet the trigger signal can barely be influenced, the generation of the primary electron pulses has to be very widebanded. This, however, encounter considerable problems if working with very short primary electron pulses desired for attaining high time resolution.
The object of the present invention is to decouple triggering and primary pulse generation, and thereby making it possible to use narrowbanded arrangements for generating even very short primary pulses.